adsl2+ & 21c

I'm not that happy with my broadband speed as its not as fast at it was in my area and throughout the town I live in. When the exchange was upgraded to 8mb it was great for a few months then everybodies speeds dropped by about a half! BT claimed no problems, ISP's no problems yet every person I know has experienced the same thing, Different ISPs different areas of the town and its never recovered. I see on the web that the exchange is due to go live with BTs 21c adsl2+ on January 31st next year with speeds "up to" 24mb. Could that have anything to do with it?Will this make much noticeable difference to ordinary users speeds? Will the costs increase via the isps? Anybody got experoience of a 21c enabled exchange?